westes

The Hydrology of Bottomless Containers / Raised Beds

westes Zone 9a California SF Bay
12 days ago
last modified: 12 days ago

I want to understand the hydrology of a raised bed / bottomless container planted over the native soil. After understanding the hydrology of a raised bed, I want to ask about ideal soil mixes for such a raised bed.

I think it helps in this discussion to first understand the movement of water in a closed container. The use of very structured soil - with large particles and organic components not taking more than about one-third of the soil mix - is all about minimizing the perched water table and allowing roots to have the best opportunity to breathe oxygen. There are many threads on Gardenweb regarding this subject, and @tapla has hundreds of posts explaining how these issues guided his development of "the gritty mix" and "5-1-1 mix". In spite of all these posts, I find very few gardeners seem to understand these issues. Everyone creates container soils with a lot of compost, manure, peat, and other fine particles whose hydrology in a closed container will drown roots and significantly raise the perched water table. Ironically, the very worst offender for this appears to be the nursery industry itself, where plants are delivered to customers in pots that contain sawdust, compost, manure, and peat moss. These are virtually bogs and unsuspecting plant owners kill these plants when they overwater the container over some period of time.

Having discussed closed containers, let us contrast this to the hydrology of raised beds/bottomless containers. My current understanding of raised beds / bottomless containers is that the hydrology is tied to the earth under the raised bed. The earth will act as a wick and draw out the moisture in the raised bed soil mix. This means that raised bed soil mixes can contain significant amounts of material that is moisture retentive like unfinished compost. In spite of the fact that this material holds too much water, the excess is being wicked away by the earth.

What are the reasons to load a raised bed with unfinished compost and manure? Presumably, the advantage is that you give the plant nutrition for its first six months, and you also enable the continual provision of organic nutrients (compost and manure) at the surface of the soil mix. Unlike a closed container, where organic nutrients can affect the soil's ability to drain, in a raised bed that is no longer a concern.

Assuming the idea of "wicking" is correct, then what are some ideal soil mixes for raised beds?

In this thread, @tapla argues that a raised bed might use 60% native soil and 40% compost. Would that be unfinished compost? I ask this because one of @tapla's other posts on this topic makes the point that finished compost does not have much nutritional value for plants (so then why add it at all?).

What percentage of the soil mix could be composted manure?

It is interesting in that thread that @tapla suggests using "builder's sand" as a replacement for the 60% native soil if your native soil is not good. The builder's sand would be a distribution of particle sizes from 1/4 inch granite down to the fines.

I assume that if you had a requirement for acidity - think rhododendrons or camellias - then you could add some peat moss into the mix? Are there any guidelines on the amount of peat moss to tolerate in a raised bed soil mix?

Can someone think of reasons to not add Turface into the soil mix for a raised bed? It would help to retain more moisture in the soil and release it more slowly, rather than having it all wicked away to the earth.


Comments (19)

  • gardengal48 (PNW Z8/9)
    12 days ago

    First, It is important to understand that a raised bed is really no different from planting directly in the ground and the same rules apply. It is just a elevated mass of soil, whether it is confined with solid sides or just a loose, mounded area. The same sort of soil that works for direct to the ground plantings will work for a raised bed as well - eg., mineral based garden soil or plain old dirt :-) And as the same as planting directly in the ground, you want to keep additions of compost or other organic matter to no more than 10% of the total volume. Same with any peat moss - no more than 10% of the total volume. No turface, no vermiculite, no perlite, no potting mix. They are unnecessary to use outdoors in either an inground setting or for raised beds.

    This is not a situation that calls for a structured or engineered soil. Just a lightly amended garden soil. Don't need to make it any more complicated than that!

    westes Zone 9a California SF Bay thanked gardengal48 (PNW Z8/9)
  • armoured
    12 days ago

    The sources I've seen that state that organic materials should be up to 10% say "dry weight", not volume. I believe this is also the number that is given when soil test results are taken. Now caveat this: I'm not claiming I've seen all the possible sources or done an extensive review, just the ones I've seen, so I may well be wrong.

    What might that be in volume terms? Well, compost - even relatively "finished" compost - is going to have more structure, air, and water in it. Also, unless the compost is really and truly "stable" - i.e. mostly done composting and bound in compounds that don't break down much normally - it will break down and lose mass. Even digging up "stable" compost and mixing it and exposing it to air etc will tend to break it down more. So even by volume - without the dry weight issue - the amount of organic materials will fall, possible fairly rapidly.

    So I don't have a number but I would think one has to start with more than the 10% target and I don't think having 25-40% compost or compostable materials in a mix by volume is outrageous to start. Right away the dry weight is probably less than half the raw number by volume, and will decline over time.

    Happy to hear other's views, though.

    Small additional thought, part of the goal of adding compost etc is not just 'fertilizer' but soil life, and providing food for soil life - even a little extra to start - is part of the goal (and quite different than a container environment).

    westes Zone 9a California SF Bay thanked armoured
  • gardengal48 (PNW Z8/9)
    12 days ago

    Armoured, please read this: Myth of Soil Amendments. And in particular please take note of the first bullet point under 'Bottom Line" : "Ideal soils, from a fertility standpoint, are generally defined as containing no more than 5% OM by weight or 10% by volume"

    westes Zone 9a California SF Bay thanked gardengal48 (PNW Z8/9)
  • Richard Brennan
    12 days ago

    A raised bed is not a container. It is dirt in the ground. It is just like planting in a field or other garden bed. It is just a few inches higher than the ground around it and there are barriers to keep the soil from washing out of the bed.


    A container is an artificial environment that is isolated from natural soil biology. Kind of like animals in a zoo, everything plants need have to be provided for them because you can't depend on the necessary interdependent species and weather wandering by.


    So, giving advice about a raised bed is no different than any other home garden situation. About the only real difference is that you can probably infer that no one plans to walk on that soil, so compaction won't be an issue.

    westes Zone 9a California SF Bay thanked Richard Brennan
  • tropicofcancer (6b SW-PA)
    12 days ago

    I would like to disagree here somewhat. Agreed that raised beds will work with native soil and some organic matter. But from a science/physics point of view there is nothing to stop you from using components from structured soil in a raised bed. You can use any number of durable material in a raised bed - granite, turface, perlite, sand, etc. And any number of organic material like bark, peat, compost etc. Organic material will decompose and the soil will sink depending on how much you add. The difference is in choice of components, ratios and particle sizes and what each component does or contributes in relation to others.

    Since Perched water and drainage is not a concern in a raised bed, you will have to look at those components differently. In general, you will have to use relatively more finer material so water retention can be kept high and still have good air porosity. I recycle some of my used gritty and 511 to my vege beds mixed in with native soil. At this point the top 6 - 8 inches is probably less than 30% native soil. It is very productive soil.

    I do not have particular ratios or recipes but Al has written about the composition of his raised beds (in the passing) off and on. Eg: https://www.gardenweb.com/discussions/5431978/best-soil-media-for-very-large-outdoor-containers#n=48

    I cannot find where Al first stated/discussed the above. This is quote from a quote from the above thread:
    "If you decide you'd like to build the soil, I'll send you a photo of
    the soil I have in my raised beds. It is a mix of pine bark, peat,
    swimming pool filter sand, crushed granite, and Turface (a baked clay
    granule). The soil is 5 years old and yields fantastic growth and offers
    great aeration and superb drainage with very good water and nutrient
    holding ability. I've done nothing to it since building the beds 5 years
    ago aside from side-dressing the plants in it with Milorganite
    periodically. The only component that would take some searching is the
    Turface, but there will be several places near you that sell it. The
    components are all inexpensive & the soil will likely end up costing
    the same as or a little less than the commercial mix you're looking
    for."

    westes Zone 9a California SF Bay thanked tropicofcancer (6b SW-PA)
  • westes Zone 9a California SF Bay
    Original Author
    12 days ago

    @gardengal48 so you would amend Al's raised bed soil formula to be something like 90% of soil mix from your regular garden soil (assuming it is not bad soil), and add 10% as compost?

  • gardengal48 (PNW Z8/9)
    12 days ago

    No one say you can't use those ingredients! There is just no need to and they are without question going to cost more than just using some slightly enriched garden soil or topsoil. And they won't necessarily work any better.

    I have known Al a long time and agree with him in most cases. But unless you have all those "structured" soil ingredients hanging around just waiting to be used, you are not going to convince me that "the soil will likely end up costing the same as or a little less than the commercial mix you're looking for."

    A standard 4'x8' raised bed filled 10 inches deep will take right around one cubic yard of material to fill. That would run $32 for the premium garden mix (sandy loam and compost combo) from the local bulk supplier. There is no way a cubic yards of all those other ingredients will cost less....in fact, they will very likely cost 3-4 times more!!

  • westes Zone 9a California SF Bay
    Original Author
    12 days ago

    So I would make the point that a raised bed is not exactly like garden soil. The hydrology is different, by virtue of the ground soil wicking away water from the raised bed. That hydrology is important to understand because it lets you "get away" with using more moisture-retentive soil mixes in the raised bed than would work well in the ground. That is not an argument for more than 10% of the soil as compost. The point is a raised bed is more forgiving of moisture-retentive soil mixes because the hydrology forces better drainage.

  • westes Zone 9a California SF Bay
    Original Author
    12 days ago

    @gardengal48 Thanks for sharing Dr Chalker-Scott's article. I always like reading her position papers. That particular paper brought together a lot of key ideas for me and was perfect to this discussion.

  • gardengal48 (PNW Z8/9)
    12 days ago

    It is just as much a case of simple elevation and the force of gravity as it is the ground soil wicking away the excess water. Very little to do with hydrology at all. And that wicking action happens just as well with non-raised bed inground plantings. A drier media will absorb or draw water from a neighboring damper one. There is also nothing anymore particularly water retentive about raised bed soil than there is with a regular inground soil that has been amended with the same amount of OM. So yes, it is exactly like garden soil!!

    westes Zone 9a California SF Bay thanked gardengal48 (PNW Z8/9)
  • westes Zone 9a California SF Bay
    Original Author
    12 days ago
    last modified: 12 days ago

    @tropicofcancer Just a caution that the thread you linked is a container thread. The comment you quote though does sound like it was an off-topic reference to a raised bed soil, but if you look at the person quoting Al, the context given was for a container, not a bottomless container. The soil mix you give sounds suspiciously like a container soil mix, and it contradicts the passage from Al that I quoted in my original post. Al was proposing 60% native soil and 40% compost, which if you use the metrics of native soil ratios given by soil science, might be modified to 90% native soil and 10% compost. Better to add nutrients from that point forward into the topsoil.

    I would love to have @tapla come here and give us the exact proportions of the components in his raised bed mix. Also, he could confirm that his use of the term "raised bed" in that quote was for a raised bed with a bottom, which is effectively a container.

  • westes Zone 9a California SF Bay
    Original Author
    12 days ago
    last modified: 11 days ago

    @gardengal48 Imagine that the water table for the ground soil is 12 inches under the soil line. The drier soil on top tries to wick the water up, but it is unable to overcome the force of gravity. At some point of balance, the adhesion and cohesion forces in the soil balance gravity, hence you get a water table that is lower than the soil line.

    In the case of the raised bed, gravity and the wicking by the drier ground soil reinforce each other. Gravity is pushing water to the ground soil. The ground soil is additionally wicking the water from the raised bed because the ground soil is drier at the soil line.

    Maybe we can at least agree that raised beds drain better than the same soil in the ground, and gravity is an important part of that.

  • tropicofcancer (6b SW-PA)
    11 days ago

    @westes: That is an extreme argument. If water table is 12 inches below surface it is close to a bog like situation. You would not want to situate your home in an area like that would you? Most residential areas and even farm lands the water table will be well below that. In such a case the difference will be marginal. Earth is a huge sink, infinite compared to a raised bed. The difference between the top 12 inches of ground and 12 inches of soil in raised bed is of no consequence.

    @westes: That quote is about raised beds that happens to be in container forum just like you posted your question on raised beds in container forum. Just because similar ingredients are used does not mean it is not suitable for raised beds. The difference is in proportions and particle sizes to make it similar and not dramatically different from ground underneath. I have many potted plants especially the larger ones partially dug into the ground . The soil in container are in intimate contact with the ground and thus act like mini raised beds. The growth of the plants is truly remarkable. It keeps the roots cooler, needs less watering and overall the plants come out stronger and robust.

    @gardengal: Agreed that a custom mix of bark, turface etc like what Al described is not going to be cheaper than bulk soil from garden supply. But I cannot agree that it will not be any better. Even in nature soil can be dramatically different in composition depending on location and each have their own characteristics suitable for certain types of plants more than others. After all the same criteria applies. Roots needs water, air and nutrients. And by adding bark, turface, grit, sand to regular soil, I can definitely make it better with respect to all three especially air. Granted it is not a feasible solution in large scale but for a backyard gardener it is quite doable considering the amount of money already spent on plants.

    On a large scale I know for sure that golf courses amend their soil with turface

    westes Zone 9a California SF Bay thanked tropicofcancer (6b SW-PA)
  • westes Zone 9a California SF Bay
    Original Author
    11 days ago

    @tropicofcancer It does not matter whether the water table is 1 ft, 5 ft, or 10 ft under the ground level. I was not arguing in favor of any specific value for a water table. I was trying to illustrate how physics (gravity) and hydrology reinforce each other in the case of a bottomless container/raised bed.

    When you plant your containers under the soil line, are you cutting out the bottoms to make them bottomless?

    When you plant a structured soil in a bottomless container, are you fertilizing that with synthetic fertilizer, or are you using organic products?

  • tropicofcancer (6b SW-PA)
    10 days ago

    It absolutely does matter where the water table is in the ground. The water table is the very definition of perched water in the case of ground. Close to a pond or water body the water table is close to the surface. As you get to a higher ground the water table is well below the surface.

    Let us take what you said: In the case of the raised bed, gravity and the wicking by the drier
    ground soil reinforce each other. Gravity is pushing water to the
    ground soil. The ground soil is additionally wicking the water from the
    raised bed because the ground soil is drier at the soil line.

    There is no such reinforcement as you think it is. This is not how it works. Wicking downwards and gravitational pull is exactly the same thing. If there was no gravity (as in space) water will not be able to move downwards no matter how much soil is there. In fact wicking downwards is wrong terminology. Wicking is always an upward action caused by adhesion/cohesion (capillary forces) forces between water and soil particles. So there are only two counteracting forces on water: gravity pulling down and capillary action pulling up.

    When you plant your containers under the soil line, are you cutting out the bottoms to make them bottomless?

    There is no need to. As long as the soil underneath is in intimate contact with the soil in the container through even one drainage hole the container will act like a raised bed. I usually use reserve this trick for a potted plant that needs a repot but cannot get to it. At that point the air/water porosity of the container soil is degraded but digging in the ground alleviates the problem and gives me more time. If space permits I will dig in other pots too. The bottom of the pot should be well clear of the compacted soil which in my case is 12-18 inches below the surface.

    For potted plants I always use synthetic fertilizer. For real raised beds organic fertilizers.

    westes Zone 9a California SF Bay thanked tropicofcancer (6b SW-PA)
  • westes Zone 9a California SF Bay
    Original Author
    10 days ago
    last modified: 10 days ago

    @tropicofcancer, I understand that the perched water table in real soil is maybe not near the surface. Understanding what that exact depth is - or should be - had little to do with my main point. The issue I was raising was that the adhesion/cohesion forces balance gravity somewhere below the level of ground soil. The soil at the ground is therefore dry.

    I agree with you that "wicking" is nothing more than adhesion and cohesion forces at work. However, you are definitely not right that cohesion and adhesion can only work against gravity. Take any paper towel and move a tiny amount of it into a pool of water. Cohesion and adhesion will work to wick the water into the paper towel, and this is true whether the towel points up or points down from the pool of water. In the case where it points down below the dish of water, the wicking effect happens faster, because gravity and cohesion/adhesion are working together instead of against each other.

    When we talk about the "water table", I usually think of a place more than 50 feet underground where the soil is completely saturated by water. Isn't there a separate kind of perched water table near the soil surface? After a rainstorm, I imagine there is a kind of transitory perched water table in the ground, which will slowly change over the course of hours or days. But that perched water zone is definitely something different than the permanent water table deep underground? Understanding this has no effect on the main discussion here. But I would like the education.

  • tropicofcancer (6b SW-PA)
    10 days ago
    last modified: 10 days ago

    Water moving through the soil is akin to diffusion. Capillary action is specially reserved for water moving upwards through the pores. I did not say molecular forces can only work against gravity. Gravity will only change the speed of diffusion not the amount diffused. If gravity was halved may be it will take twice as long but the same amount of water will pass through the soil given the same amount of water is added. A part will be held by the soil and the rest will pass through.

    Perched water in the ground is the water table height. That is where the name perched water comes from in the first place. In the case of ground, the composition can dramatically vary creating a variety of configuration of how and where the water is held. If you look up aquifers you will see what I mean. The water table height itself can vary up or down depending on rain and other seasonal variation. USGS maintains thousands of wells to monitor the water table table height at various locations.

    After a rain storm or flood water a saturated zone of water will move down through soil but will not be called perched water. Perched water has specific definition - a zone of permanent saturation unless another external force acts on it to change it.

    Permanent to the extent of permanent features of ground. Obviously, it changes up/down in response to weather and other factors.

    A saturated zone is where the airspace between particles is saturated. Above that air space cannot hold water against gravity. All the water is held by adhesive forces between water and soil particles. Let us say 1 inch above the saturated zone has X amount of total water. Your contention is that 10 feet above it is less than X. Why should it be? What has changed between 1 inch and 10 feet? The gravitational force is not much different at the surface or say 10 feet below. (technically it is slightly lower but very insignificant). Nothing else has changed, it is the same soil with same molecular forces at work. So water content at the surface should be the same as 1 inch above the perched water level. Discounting the effect of Sun evaporating the water from the surface. Temperature is the only factor that will change but has insignificant effect on within the range we are talking about. Assuming the soil composition does not dramatically change either.

  • westes Zone 9a California SF Bay
    Original Author
    9 days ago
    last modified: 8 days ago

    @gardengal48 Regarding the myths of soil amendments and the generalization about 5% compost in native soils:

    * Do you have any other references? What I would love to see is a table of different locations throughout the world, showing the type of soil they have (e.g., loam, clay, sand) and the percentage of organic matter in the soil. I actually had someone read the article from Dr Chalker-Scott and claim that only applies to Washington State. :) I went to Google more references and I must have been using the wrong keywords.

    * Does the percentage of compost in the soil that might be beneficial change in a vegetable garden? That's where you see a lot of compost used. But is the safest use of that compost as a top dressing, and the guidance would still be to keep the soil itself around 5% compost? Would the amounts and frequency of surface compost application be larger/more frequent than other types of plants?

    * In an agricultural setting (i.e., a farm), how are they conditioning soil before planting? After testing the existing farm soil, are they tilling in compost to bring the soil levels to about 5% organic matter content?

    * If you get the soil to levels of compost that are much higher than 5% to 10%, what kind of harm to the plant can result? Is it only about root rot, or are there other diseases that the high compost levels will make the plant subject to?

  • westes Zone 9a California SF Bay
    Original Author
    7 days ago
    last modified: 6 days ago

    @tropicofcancer, I am not claiming that there is a stronger downward force 10 feet above the saturation point of water in the ground than one inch above the saturation point. I understand your point is that the forces in a raised bed will be identical to the forces at the soil surface. When things are dry at the soil surface, I agree, but this is not always the case.

    Many gardens have spots where the soil at the surface saturates with water and becomes too wet to support certain types of plants, such as cactus or succulents. In my garden, I have a spot where cactus thrive in the ground, but just six feet away is a spot that gets completely saturated with water and seems to want to hold it longer. Cactus planted in that spot develop root rot. So for such areas raised beds will help a lot, and they will give you a soil mass that will drain better.

    I still maintain that cohesion and adhesion forces can work in all directions, both opposing gravity and aligned with it. But that point is not important to this discussion. As long as I am addressing an area in my garden where the soil tends to stay wet at the surface, a raised bed will give additional drainage as compared to the soil surface.

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